This invention relates to a high yield process for the production of methyl methacrylate (xe2x80x9cMMAxe2x80x9d) or methacrylic acid (xe2x80x9cMAAxe2x80x9d) and an apparatus for increasing the yield in a process for the production of MMA or MAA.
A number of commercial processes are used to prepare MMA. In one such process, MMA is prepared from acetone cyanohydrin (xe2x80x9cACHxe2x80x9d). The process is described in U.S. Pat. No. 4,529,816 (xe2x80x9c816xe2x80x9d). In this process, ACH is (1) hydrolyzed by sulfuric acid to produce xcex1-hydroxyisobutyramide (xe2x80x9cHIBAMxe2x80x9d) and its sulfate ester, xcex1-sulfatoisobutyramide (xe2x80x9cSIBAMxe2x80x9d); (2) the HIBAM and SIBAM are thermally converted to 2-methacrylamide (xe2x80x9cMAMxe2x80x9d) and a small amount of methacrylic acid (xe2x80x9cMAAxe2x80x9d); which are then (3) esterified with methanol to produce MMA. Residual HIBAM is esterified to methyl xcex1-hydroxyisobutyrate (xe2x80x9cMOBxe2x80x9d). In step (2) of the reaction, the conversion of SIBAM to MAM occurs more readily than the conversion of HIBAM to MAM. In order to facilitate the thermal conversion of HIBAM to MAM, both heat and increased residence time must be provided. A decrease in thermal conversion to desired products results in a decreased overall yield for the process. The process of preparing MAA can be the same as that used to prepare MMA, except that instead of esterifying MAM and MAA with methanol, water is added to the MAM and MAA mixture to convert the MAM to MAA.
The MMA and MAA markets are extremely cost sensitive. A slight improvement in process yield can result in a significant market advantage. There is a need for an improved yield commercial process of preparing MMA or MAA.
One attempt to improve the yield of a MMA process is disclosed in U.S. Pat. No. 5,393,918. In addition to the conversion of MAM to MMA, the patent discloses a process whereby the HIBAM and SIBAM from step (1) above are esterified to methyl-xcex1-methoxyisobutyrate (xe2x80x9cxcex1-MEMOBxe2x80x9d), methyl-xcex2-methoxyisobutyrate (xe2x80x9cxcex2-MEMOBxe2x80x9d), and methyl xcex1-hydroxyisobutyrate (xe2x80x9cMOBxe2x80x9d). The xcex1-MEMOB, xcex2-MEMOB, and MOB are later isolated and converted to MMA in a separate step. This eliminates the need for thermal conversion of HIBAM and SIBAM to MAM, but requires fractional distillation to separate out xcex1-MEMOB, xcex2-MEMOB, and MOB from MMA and a subsequent dehydration step to convert xcex1-MEMOB, xcex2-MEMOB, and MOB to MMA.
Thermal conversion of HIBAM and SIBAM to MAM typically is performed in a cracker reactor. The cracker reactor contains a heat exchanger to provide the heat necessary for the cracking reaction and a thermal conversion apparatus which provides the necessary retention time under the heated conditions for the cracking reaction to take place. A typical thermal conversion apparatus 1 of a cracker reactor known in the art is a multi-pass metal pipe (FIG. 1). In one embodiment, the metal pipe may contain a baffle 2 which separates the pipe to provide a passage 3 having a 180xc2x0 turn 4 to minimize the space required to house the cracker reactor, an expansion 5 where the reactants enter the thermal conversion apparatus, and a constriction 6 where the cracker reactor mixture exits the thermal conversion apparatus. These features of a typical thermal conversion apparatus result in backmixing of the HIBAM, SIBAM, MAM, and MAA. Backmixing of these components results in less than plug flow and decreased overall yields because the retention time of the components in the cracker reactor will vary. Some of each component will have insufficient retention time in the cracker reactor, while another portion of each component may have an extended retention time in the cracker reactor. As a result of insufficient retention time in the cracker reactor, there may be an under conversion of HIBAM. As a result of an extended retention time in the cracker reactor, there may be an over conversion or degradation of SIBAM, MAM, and MAA.
U.S. Pat. No. 4,748,268 discloses a process for preparing methacrylic acid esters using a plug flow reactor. In the process, a feed stream containing methacrylic acid, a C1-C4 saturated aliphatic alcohol, a catalyst, and a liquid organic substance is continuously fed into a plug flow reactor. The plug flow reactor is utilized for the esterification reaction. The process does not address the conversion of HIBAM and SIBAM to MAM in a thermal conversion apparatus.
Despite the disclosure of the prior art, there is a continuing need for an improved yield commercial process of preparing MMA.
We have discovered that the use of plug flow in the thermal conversion apparatus of a MMA process significantly improves the thermal conversion of HIBAM and SIBAM to MAM, and therefore provides an improved overall process yield. By plug flow is meant that the velocity of the fluid in the pipe is nearly the same throughout the cross section of the pipe.
In a first aspect, the present invention provides a process for preparing a monomer selected from methacrylic acid and methyl methacrylate, including: (A) hydrolyzing acetone cyanohydrin to produce a hydrolysis mixture including xcex1-hydroxyisobutyramide, xcex1-sulfato isobutyramide, 2-methacrylamide, and methacrylic acid; (B) thermally converting the hydrolysis mixture in a cracker reactor including a plug flow thermal conversion apparatus with the necessary retention time to produce a cracker reactor mixture including 2-methacrylamide and methacrylic acid; (C) reacting the cracker reactor mixture and a material selected from methanol and water in at least one reactor to produce a monomer selected from methacrylic acid and methyl methacrylate.
In a second aspect, the present invention provides a thermal conversion apparatus, including: a pipe with means for maintaining plug flow.